The present invention is in the field of optical information storage and particularly relates to a new thin film optical information storage medium based on a silver halide-containing film which stores information by irreversibly darkening on exposure to intense light.
Optical recording has gained commercial interest in recent years because of its promise of extremely high information storage density. At present, the recording material favored for such use is a thin film of a tellurium-based alloy. Recording in this medium is accomplished by focusing a laser beam to a 1-micron diameter spot, burning a hole in the tellurium film and thereby increasing the transparency of the medium.
Even though high recording density and sensitivity have been demonstrated in this material, serious problems relating to the long term stability of the medium remain, due to the fact that oxidation of the medium can occur in storage. A useful comparison of the various optical information storage technologies is provided by R. A. Bartolini in the article "Optical Recording Media Review," SPIE Vol. 123 Optical Storage Materials and Methods (1977), pages 2-9.
The photosensitivity of silver halide crystals is well known and silver halide-based systems are still being investigated for possible use in digital optical information storage systems. Silver halide-based films which do not require chemical development when used as information storage media have recently been described by N. F. Borrelli and P. L. Young in two commonly assigned copending patent applications, Ser. No. 901,428 filed May 1, 1978 and Ser. No. 86,813 filed Oct. 22, 1979. The media described in these applications incorporate films which are light-absorbing as made, and which can be efficiently altered to a dichroic, birefringent state by exposure to polarized light.
The silver halide films described by Borrelli et al. are multilayer films produced by the repeated sequential deposition of thin silver chloride layers and an inorganic chemical agent which develops additive coloration in each of the silver chloride layers by a limited partial reduction of some of the silver chloride to metallic silver. The characteristic light absorption and optical bleaching properties of these films have been attributed to the presence therein of additively colored silver chloride crystals, a crystal phase thought to comprise silver metal in combination with AgCl crystals. Lead oxide is one chemical agent which has been found effective to induce additive coloration in thin silver chloride layers, producing a highly colored, efficiently alterable information storage film.
Additively colored photosensitive films of the type above described are particularly useful for applications involving reuse because they are erasable and re-recordable simply by further exposure to light of an appropriate polarization. However, the long-term light sensitivity needed for such use can be viewed as a disadvantage where archival storage characteristics are desired, since continual high light sensitivity requires that the medium with its contained information be stored away from light. Thus a silver halide optical information storage medium offering good recording efficiency and archival information storage characteristics, yet not requiring a chemical development step, would still be desirable.